NCERT Solutions for Class 9 Science Unit 12

Sound Class 9

Unit 12 Sound Exercise Solutions

Sound is a type of energy. Sound travels in the form of wave from one place to another. Sound is produced because of the vibration of any object. In other words, when an object is vibrated it produces sound.When a stone is dropped in pond water, it produces many circular wrinkles and water appears to be travelling outwards from centre. These circular wrinkles are called ripples. This happens because of formation of waves in water. When a stone is dropped in water, it creates a disturbance in water particles. The water particles pass the disturbance to the adjacent particles and the adjacent particles pass the disturbance to the next adjacent particles and so on. This phenomenon continues and the disturbance moves outward from the centre of disturbance. This makes water appear to be moving. In fact, water molecules do not travel in this case, rather only the disturbance is passed through particles of water. This creates wave in the form of ripples in water.

Question : Solutions of Questions on Page Number : 162

Q1 :

How does the sound produced by a vibrating object in a medium
reach your ear?

Answer :

When an object vibrates, it forcesthe
neighbouring particles of the medium to vibrate. These vibrating
particles then force the particles adjacent to them to vibrate.
In this way, vibrations produced by an object are transferred
from one particle to another till it reaches the ear.

When the school bell vibrates, it forcesthe
adjacent particles in air to vibrate. This disturbance gives rise
to a wave and when the bell moves forward, it pushes the air in
front of it. This creates a region of high pressures known as
compression. When the bell moves backwards, it creates a region
of low pressure know as rarefaction. As the bell continues to
move forward and backward, it produces a series of compressions
and rarefactions. This makes the sound of a bell propagate
through air.

When a sound is reflected from a distant object, an echo is
produced. Let the distance between the reflecting surface and the
source of sound production remains the same. Do you hear echo
sound on a hotter day?